In designing valve gear for internal combustion engines operating at speeds in excess of 5,000 rpm, it has been found desirable to employ valve gear of the cam-over-valve type. Valve gear of this type is known as direct-acting valve gear and employs a tappet having one end contacting the engine cam shaft with the other end of the tappet in direct contact with the end of the stem of the combustion chamber valve. Direct acting valve gear offers the advantage of low mass, fewer working parts and higher stiffness due to the elimination of the rocker arm and/or push rods. Low mass and high stiffness result in a high natural resonent frequency which allows the valve gear to attain higher rpms before valve mis-motion occurs. Direct acting valve gear also permits the use of lighter valve spring loads for a given valve motion and engine speed as compared with those used in other valve gear arrangements. The low mass and high stiffness of the system also permits valve lift velocities and accelerations which increase the area under the valve lift curve and thus provide increased specific engine output. Although other overhead cam configurations can be made to have comparable lift velocities and accelerations, a direct acting valve gear arrangement offers the additional advantage of permitting rotation of the cam contacting surfaces as the lifter rotates which is not possible with rocker arm type valve gear arrangements. Direct acting valve gear arrangements, therefore, allow higher permissible cam contact stresses.
In addition, the cam profile for other overhead cam valve gear arrangements with high lift accelerations and velocities is more complex than that required for direct acting valve gear. The simpler cam profile requirement of direct acting valve gear results in less manufacturing difficulties and less cost in the valve gear when high velocities and accelerations are desired.
It has, however, been found difficult to provide direct-acting valve gear in engine applications where the height of the engine must be kept to a minimum and consequently the cam shaft located closely adjacent the end of the combustion chamber valve stem. Furthermore, where it is desired to retrofit an hydraulic lash adjusting tappet into the direct-acting valve gear of a production engine, it is often difficult to provide a hydraulic lash adjusting tappet in the space provided between the cam shaft and the end of the valve stem. Since the tappet must be guided in a bore provided in the engine, it is necessary to provide structure intermediate the cam shaft and the end of the valve stem in order to provide the tappet guide bore and the engine height tends to somewhat increase.
Therefore, it has been desired to find an hydraulic lash adjusting tappet with a compact profile height for use in engines having direct acting valve gear with minimum distance between the cam shaft and the end of the valve stem to minimize the mass of engine structure necessary to provide the tappet guides. Furthermore, in designing tappets for direct acting valve gear so as to minimize side loading in the guide for minimizing wear, it is desirable to have the reaction force of the valve stem centered through the tappet at a point as closely adjacent the cam surface as possible. Locating the reaction force near the cam face also permits the tappet to be designed to minimize the mass, which in turn reduces inertia.
Known hydraulic tappets for direct-acting valve gear have employed a body, or bucket, formed as an integral unit having a reservoir provided by an undercut in the plunger guide bore formed in the bucket, such as that shown and described in U.S. Pat. No. 3,509,858 to Scheibe et al, wherein the necessity of undercutting requires a relatively large plunger guide bore in the body which in turn results in a reduced hydraulic pressure upper operating limit. Furthermore, if the diameter of the plunger guide bore is reduced, the undercut reservoir is reduced in volume and the mass of the tappet is increased, resulting in greater inertia.
Another problem encountered in designing hydraulic lash adjusting tappets for direct-acting valve gear has been the problem of providing means for retaining the hydraulic lash adjusting plunger sub-assembly in the tappet bucket prior to installation in the engine.
A further problem encountered in the design of such hydraulic bucket tappets has been the necessity of providing precision sliding surfaces on the outer diameter of the plunger and the inner periphery of the plunger guide bore formed in the bucket. Such precision surfaces are required in order to provide control of leakdown from the high pressure hydraulic chamber in the tappet where this sliding interface is employed as the leakdown control surface, as, for example, in the tappet described in German Pat. No. 1,914,693. In tappets having this latter known construction, the high pressure hydraulic fluid chamber for effecting lash adjustment is disposed at or near the upper level of the fluid reservoir and consequently is susceptable to retention of trapped air. This requirement for leakdown control has heretofore required extremely tight tolerances on the dimensions of the bucket bore and plunger diameter. The necessity of forming the plunger guide bore in the bucket to tight tolerances has resulted in costly scrap losses if such operations are defectively performed.